It is well known that each cell within an organism contains the genetic information that encodes all of the proteins found within that organism. However, only a very small percentage of the genes present within a given cell type is actually transcribed. The intracellular mechanisms that regulate the array of genes to be transcribed are now understood. Cell specific proteins present within the nucleus interact with DNA regulatory Segments that are linked with particular genes. This interaction of nuclear proteins with DNA regulatory sequences is required for gene transcription. This results in mRNA biosynthesis and ultimate expression of the encoded protein (Mitchell and Tjian, Science, 245:371, 1989).
These DNA regulatory segments or elements for each gene lie upstream from and, in some cases, within or even downstream of the coding regions. Through an interaction with cell specific nuclear proteins, DNA regulatory segments affect the ability of RNA polymerase, the rate limiting enzyme in protein expression, to gain access to the body of the gene and synthesize a mRNA transcript. Thus, these DNA segments and the resident nuclear proteins play a critical role in the regulation of expression of specific genes (Johnson and McKnight, Ann. Rev. Biochem., 58:799, 1989).
The DNA regulatory segments are binding sites for the nuclear proteins. These nuclear proteins attach to the DNA helix and apparently alter its structure to make the desired gene available for RNA polymerase recognition, which facilitates gene transcription. The expression of these cell specific regulatory proteins determines which genes will be transcribed within a cell and the rate at which this expression will occur. As an example of the specificity of this system, pituitary cells but not liver cells express pituitary proteins, even though the genes for the pituitary proteins are present within all liver cells. Nuclei of the liver cells do not contain the specific DNA binding proteins which interact with the elements of pituitary genes resident within the liver cells.